Exporting Images/Movies to Final Cut Pro X to apply LUTs
If you are trying to master for HDR displays, you'll need to start ideally from a scene referred encoding such as an EXR. This likely is a bit of a mess in FCPX if I had to wager, as most applications are mostly designed for display referred work.

LUTs are tricky things. That is, any LUT is designed for a very specific input and a very specific output. Trying to modify imagery with LUTs designed for other contexts is a bit of a messy ball of nightmare.

It is hard to offer any further help as the question lacks important details and context.